Typical of the external features required on a casing are bosses. These are locally thick protrusions which facilitate the bolting of pipes, bleed valves and the like to the casing as required by internal machinery enclosed in the casing. A typical boss protrudes from an outer wall of the casing in an annular shape defining a through hole to the inside of the casing and an array of bolt holes encircling the through hole. Additional components such as pipes, valves and the like typically have flanges which match with the boss annulus and these components are secured to the casing by bolts passed through the flange and the boss.
In the particular case of gas turbine engines, casings must be able to withstand high loads and extremes of temperature and pressure. It is known to manufacture such casings from high performance alloys using a powder hot isostatic processing (PHIP) process.
In the PHIP process, coaxially aligned steel canister portions are arranged to define the geometry for the casing wall between them. To provide a boss on the outer wall of the casing, a shape defining the boss geometry is cut into a radially inner wall of a radially outer canister portion. High performance alloy powder is poured into the space between the canister portions under vacuum. The canister is then sealed, placed in a pressure vessel and heated to a high temperature in conditions of high pressure. This causes the powder to amalgamate into a solid structure having the geometry defined by the opposite facing walls of the canister portions. The canister portions can then be removed from the product, for example by machining and/or acid etching. Due to the high pressures imposed during the process, the resulting product dimensions are relatively smaller than the starting dimensions defined by the canister portions and its material very dense. The product at this stage is known as a nett shape PHIP condition of supply or PHIP COS. In order to make the finished casing, surfaces of the PHIP COS which, in use, will interface with other components are finished with appropriate machining processes. The process is cost effective minimising use and wastage of the expensive high performance alloy powder.
The minimum required height of the boss relative the casing surface is defined by two factors; firstly, it must be sufficient to meet the stress requirements on the boss when the casing and associated components are put to their intended use. Secondly, the boss and casing together must provide a sufficient depth to accommodate a thread length needed to receive bolts which attach components interfacing with the boss. It is not unusual for the thread length requirement to dictate a greater dimension than the stress considerations. For this reason, boss height across the entire boss exceeds the minimum height required for stress considerations. In some alternatives, the boss height is at a minimum for stress conditions but the entire casing wall is made thicker in the region of the boss to accommodate the required bolt threads.